1. Field of the Invention
The present invention relates generally to the small-scale simulation of crude oil refinery desalters, free water knockouts and heater treaters, and more particularly, to a dynamic desalter simulator that enables the direct observation of the rag layer at the crude oil and water interface and the ability to remove samples of the fluids or emulsions during the simulation and monitor parameters such as temperature, pressure, conductivity, chloride, and pH.
2. Description of Related Art
Liquid hydrocarbon phase, such as crude oil, naturally contains a variety of contaminants that have detrimental effects on process equipment and in the operation of a refinery. These contaminants are broadly classified as salts, bottom sediment, water, solids, and metals. The types and amounts of these contaminants vary depending on the particular hydrocarbon phase. Additionally, native water present in the liquid hydrocarbon phase as droplets may be coated with naturally occurring surfactants such as asphaltenes, naphthenic acid salts, resins, or with solids including but not limited to iron oxide, silica, carbon, carbonates, or phosphates. Removing the water from the crude oil is essential at crude oil production facilities as it impacts the value of crude oil and its economic transportation. The presence of salts, especially chlorides of Group I and Group II elements of The Periodic Table of Elements, causes corrosion of oil processing equipment. In order to mitigate the effects of corrosion, it is advantageous to reduce the salt concentration to the range of 1 to 5 ppm or less and water content to about 0.10 to 1 wt % by weight of the crude oil prior to transportation and processing of the oil.
A standard treatment for removing small particles of solids and bottom sediment, salts, water and metals is a phase separation operation commonly known as dewatering or desalting. A fresh water wash in the range of typically 4 to 15 vol % is injected into the crude oil. The crude oil and wash water are subjected to shear to thoroughly mix the water and the crude oil to form an emulsion and to transfer the contaminants from the crude oil into the fresh water. Frequently a chemical emulsion breaker is also added to the emulsion, and often the emulsion is subjected to an electrostatic field so that water droplets in the mixture of crude oil, wash water, and chemical emulsion breaker coalesce in the electrostatic field between electrodes. The coalesced water droplets settle below the oleaginous crude oil phase and are removed. The treated crude oil is removed from the upper part of the separator.
One problem encountered with dewatering and desalting is that some crude oils form an undesirable “rag” layer comprising a stable oil-water emulsion and solids at the water-oil phase boundary in the desalter vessel. The rag layer often remains in the vessel but it may be removed for storage or for further processing. Rag layers at the water-oil phase boundary result in oil loss and reduced processing capacity. Heavy crude oils containing high concentrations of asphaltenes, resins, waxes, and napthenic acids exhibit a high propensity to form rag layers.
Additives may be added to improve coalescence and dehydration of the hydrocarbon phase, provide faster water separation, improve salt or solids extraction, and generate oil-free effluent water. These additives, also known as demulsifiers, are usually fed to the hydrocarbon phase to modify the oil/water interface. It is also possible to feed these materials to the wash water or to both the oil and water. These additives allow droplets of water to coalesce more readily and for the surfaces of solids to be water-wetted. The additives reduce the effective time required for good separation of oil, solids, and water.
Development of new chemical demulsifiers has typically been done using static simulations. In the past, static tests using a simple apparatus such as glass bottles or glass tubes and are referred to as “bottle testing”. These methods have proven to be useful but they often fail to adequately simulate many critical parameters of a desalter and have been of limited use particularly in heavy oils or systems that have a propensity to develop rag layers. In particular, bottle testing fails to simulate the development of a rag layers since these develop over time in dynamic systems that continually refresh the oil.
In the simplest embodiment, oil samples with treatments are added to prescription bottles and shaken. The rate of demulsification (water removal) is then monitored as a function of time by observing the amount of “free” water that collects at the bottom of the bottle. This simulation is sufficient for desalting vessels that are operated at low temperatures with light oils. However, it fails to correctly simulate the size of the water drops in the actual emulsion and the actual temperature. The equation below is Stokes Law and describes the settling velocity of spherical water drops in a fluid.
      V    s    =            2      9        ⁢                            r          2                ⁢                  g          ⁡                      (                                          ρ                p                            -                              ρ                f                                      )                              η                      where:        VS is the particles' settling velocity (cm/sec) (vertically downwards if ρp>ρf, upwards if ρp<ρf),        r is the Stokes Radius of the particle (cm),        g is the gravitation constant (cm/sec2),        ρp is the density of the particles (g/cm3),        ρf is the density of the fluid (g/cm3), and        η is the fluid viscosity (dyne sec/cm2).        
The simplistic bottle shaking simulations often fail to correctly simulate the drop diameter and the temperature and viscosity (η), which is a function of temperature. This results in data that fails to properly describe the system being studied. For heavier oils, the bottle shaking simulation is even worse. Actual desalting temperatures are between 120° C. to 150° C., and these simple simulations are not close to actual conditions of viscosity, drop size and in materials like SAGD fluid or Venezuelan Extra Heavy crude since the density difference between the water and the oil is significant at the higher temperatures. Thus, this driving force for separation is incorrectly simulated. Historically, people add diluents, a light aliphatic hydrocarbon, in efforts to better simulate the viscosity and density differences, but this changes the polarity of the oil and can precipitate the asphaltenes and completely change the nature of the oil. Without the addition of the diluents to reduce the viscosity, the separation time at low temperatures can be many hours.
Dynamic test simulators that allow actual operating conditions of a desalter to be used in the experiments have been built. These devices include “pilot” units that use hundreds of barrels per day to laboratory systems that use 50 to 100 gallons of oil per day. These devices still use sizeable quantities of crude oil, have limited abilities to modify system parameters, and the ability to observe the development of a rag is not possible.
It is desired to improve simulation methods such that one may select the most efficacious chemistries and operating conditions to optimize the emulsion breaker chemistries, oil mixtures, temperatures, emulsion size, and other parameters.